The present invention relates to optical systems. In particular, the present invention relates to optical systems in data storage devices.
Optical data storage systems access data by focusing a laser beam or other light source onto a data surface of a medium and analyzing the light reflected from or transmitted through the medium. In general, data is stored in optical storage systems in the form of marks carried on the surface of the medium which are detected using a reflected laser light.
Compact discs, which are widely used to store computer programs, music and video, are one type of optical data storage system. Typically, compact discs are permanently recorded during manufacture by etching the surface of the compact disc. Another type of optical system is a write once read many (WORM) system in which a user may permanently write information onto a blank disc. Other types of systems are erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems read data by measuring the rotation of the incident light polarization due to the magnetic state of the storage medium.
Each of the above systems focuses a beam of light on to one of a plurality of concentric data tracks on the surface of the disc. To position the beam, most systems use a coarse actuator that positions an objective lens over the desired data track. Light is then reflected through a series of mirrors so that it passes through the objective lens and forms a focused spot on the desired data track.
In some systems, a fine control actuator is used in conjunction with the coarse actuator to position the beam. Typically, this fine control actuator manipulates the position of one of the mirrors that passes the light beam to the objective lens. By changing the position of this mirror, it is possible to move the focused spot of light across the disc without moving the objective lens.
In current systems, the coarse and fine control actuators are controlled by reading servo information from the disc using the focused spot of light. This servo information indicates the position of the light on the disc, and based on this information, a servo controller adjusts the coarse and fine actuator to position the spot of light at the desired position.
With increasing track densities, track widths continue to decrease making it more difficult to position the light beam within a track using just servo information stored on the disc. To more accurately position the light beam, a system is needed that provides more information about the position of the light beam.